Pump assembly for power steering system

ABSTRACT

A hydraulic power steering system includes a main pump assembly which consists of two variable displacement reversible pumps of different sizes. These pumps are coupled to a common drive shaft so that they turn together and their displacements are controlled so that the output of the smaller is always a fixed proportion of the larger. A small fixed displacement hand pump is operated through means of the steering wheel and provides a flow to control the displacements of the two variable displacement pumps to place the latter in stroke. Output flow from the larger pump is connected to and provides the operating fluid for the steering motor. Output flow from the smaller pump is fed back to counteract the effects of the hand pump and return the two variable displacement pumps to their zero displacement positions once the turning of the steering wheel ceases. The amount of fluid pumped to the steering motor is thus proportional to the amount of fluid delivered by the hand pump.

Nelson 1 May 28, 1974 1 1 PUMP ASSEMBLY FOR POWER STEERING SYSTEM [75]Inventor: Roger John Nelson, Cedar Falls,

[62] Division of Ser. No. 166,202, July 26, .1971, Pat. No.

[52] U.S. Cl 417/202, 417/204, 417/216, 418/18,4l8/37 [51] Int. Cl. F04b23/10 [58] Field of Search ..4l8/3, 18, 33, 37, 38, 418/131, 241;91/496, 497; 417/202, 204,

[56] References Cited 7 FOREIGN PATENTS OR APPLlCATlONS 444,706 3/1936Great Britain 418/18 1,040,567 10/1958 Germany 418/37 Primary Examinerwilliam L. Freeh [57] ABSTRACT A hydraulic power steering systemincludes a main pump assembly which consists of two variabledisplacement reversible pumps of different sizes. These pumps arecoupled to a common drive shaft so that they turn together and theirdisplacements are con trolled so that the output of the smaller isalways a fixed proportion of the larger. A small fixed displace ment.hand pump is operated through means of the steering wheel and provides aflow to control the displacements of the two variable displacement pumpsto place the latter in stroke. Output flow from the larger pump isconnected to and provides the operating fluid for the steering motor.Output flow from the smaller pump is fed back to counteract the effectsof the hand pump and return the two variable displacement pumps to theirzero displacement positions once the turning of the steering wheelceases. The amount of fluid pumped to the steering motor is thusproportional to the amount of fluid delivered by the hand pump.

3 Claims, 5 Drawing Figures PATENTEDMAY 28 m4 SHIEU 1 BF 4 FIG. I

PATENTEDmze @914 3813188 sum 3 nr 4 I PATENTEUmes mm 3.813; 188

' sum u UF 4 CROSS REFERENCE TO RELATED APPLICATION This is a divisionof application Ser. No. 166,202,

filed July 26, 1971 now US. Pat. No. 3705493.

BACKGROUND OF THE INVENTION The present invention relates to ahydrostatic steering system and more particularly relates to a main pumpassembly for such a system.

In known hydraulic power steering systems, valving of various types isactuated either directly or indirectly by steering wheel motion toconnect a source of fluid pressure to one or the other of the work portsof a double-acting steering motor and to connect the remaining work portto exhaust. A hydraulic or mechanical feedback from the steering motoris used to return the valving to a non-actuatedcondition once thesteering wheel motion ceases. Successful operation of these systems isprimarily dependent on the responsiveness of the valve members to inputand feedback signals and this responsiveness is in turn dependent onkeeping. tolerances such that the valve members will seat and unseatproperly-and suchthat the feedback linkages are free from slack orplay." It is often very difficult and/or expensive to design,manufacture and maintain these tolerances.

Another disadvantage of these sytems is that responsiveness is oftenlost due to the necessity. caused by the SUMMARY OF THE INVENTIONAccording to the present invention, there is provided a mainpump'assembly for a hydraulic power steering system. Specifically,thereis provided a main pump assembly including a pair of variabledisplacement reversible pumps mounted as a-unit in one housing andhaving a common drive shaft, one of the pair of pumps deliveringoperating fluid to a reversible steering motor in response tobeingjplaced in stroke; by a steering wheel operated hand pump and theother of the pair of pumps delivering feedback fluid to the strokecontrol means to place the one pump out of stroke once the operation ofthe hand pump has ceased.

An object of the invention is to provide a steering system utilizing aminimum of valvingand having a very responsive feedback signal-producingand delivering means. v

More specifically it is an object to provide in such a system theaforementioned main pump assembly, the assembly being compact and havingno intermediate valving between the pair of pumps.

' A further object is to control the displacements of the pair of pumpssuch that the displacements of one is al- 1 ways a fixed directproportion of the other.

Another object is to provide a design wherein the amount of fluiddelivered by the one variable displacement pump to the steering motor isdirectly proportional to the amount of input signal fluid delivered bythe hand pump to, the one pump.

These and other objects-will be apparent from the ensuing descriptionandappended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematical view showingthe hydraulic steering system embodying the'pump assembly of the presentinvention.

FIG. 2 is a transverse sectional view of the pump assembly showing thepistons for controlling the stroke or displacement of the two pumps.

FIG. 3 is a longitudinal sectional view of the pump assembly taken alongthe line 3-3 of FIG. 2.

FIG. 4 is a transverse sectional view of the pump assembly taken alongthe line 44 of FIG. 3.

F IG. 5 is a transverse sectional view of the right-hand end cap of thepump assembly taken along the line 5-5 of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, thereis schematically shown a hydraulic power steering system indicated inits entirety by the reference numeral 10. The system 10 is illustratedas being a closed system, however, it should be understood that a sourceof fluid for making up leakage may be connected to the system in theusual manner.

The system 10 includes a main pump assembly 12 comprising a steeringfluid delivery pump 14 and a feedback fluid delivery pump 16, the pump14 being the larger of the two pumps and shown to the right of the'pump16 in FIG. 1. The pumps 14 and 16 are reversible flow, variabledisplacement pumps and are continuously driven at the same speedpreferably by the engine of the prime mover being steered, hererepresented, at 18, in block form.

The steering fluid delivery pump 14 includes a pair of intake-exhaustports 20 and 21 which are respectively connected to the service or workports 22 and 23 of a reversible hydraulic steering motor 24 by a pair offeed lines 26 and 27. As will be apparent from the description below,the pumps 14 and 16 are normally at zero stroke or displacementpositions when no steering is taking place and may be placed into strokeby selectively pressurizing the top and bottom ends respectively ofupper and lowerbores 28 and 29 in which are respectively located strokeor displacement control pistons 30 and '32. Such pressurization isaccomplished through means of a reversible flow, fixed displacement handpump 33 driven through means of a steering wheel 34 and having a pair ofintake-exhaust ports 35 and 36 respectively connected by lines 37 and 38to the bores 28 and 29. Pressurized fluid is respectively deliveredthrough the ports 35 and 36 in response to turning the steering wheelclockwise for making a right-hand turn and counterclockwise for making aleft-hand turn. The feedback pump 16 also has a pair of intake-exhaustports 39 and 40 respectively connected via passages 41 and 42, to thebores 28 and'29 for a purpose explained below.

The main pump assembly 12' appears in detail in FIGS. 2-5 and includes abox-like housing or casing 44 defining a substantially rectangularcavity 46 and including left and right side walls 48 and 50 (FIG. 3) inwhich cylindrical openings ,52 and 54 are respectively located incoaxial relationship to a horizontal axis X-X. A drive shaft 56 linesalong the axis X-X and extends axially through a bearing end cap' 58inserted in the opening 52 and terminates in the cavity 46 short of abearing end cap 60 inserted in the opening 54. A pair of circularpumping piston mounting plates 62 and 64 are splined at axially-spacedlocations on the shaft 56 for rotation with the latter and includerespective hubs 66 and 68 on which the inner races of a pair of taperedroller bearing assemblies 70 and 72 are respectively mounted, the outerraces of the bearing assemblies 70 and 72 respectively being received ina pair of thrust rings 74 and 76, which in turn are respectivelyreceived in the housing openings 52 and 54.

A hollow rectangular stroke or displacement control member 78 is locatedfor vertical movement between and has left and right sides 80 and 82(FlG. 3) in engagement with the opposed faces of the plates 62 and 64. Apair of coaxial circular openings 84 and 86 respectively in the controlmember sides 80 and 82 are concentric to and cooperate with acylindrical tube 88 loosely received on the drive shaft 56 to define acircular path 90 (FIG. 2) and confine seven pumping pistons or elements92, of the pump 14, having the shapes of cylindrical ring segments.Respectively located centrally in the seven pistons 92 are sevenhorizontal pins 94 having slippers 96 and 98 respectively pivotallyreceived on their leftand right-hand ends (FIG. 3), the slippers 96being respectively slidably received in seven radially extendingequiangularly-spaced slots 100 in the inner face of the plate 62 and theslippers 98 being respectively slidably received in seven radial slots102 in the inner face of the plate 64 in opposed relationship to theslots 100,. It is here noted that the slippers 96 also serve as thepumping pistons or elements of the feedback pump 16 as is furtherdescribed below. Forming continuations of and extending centrally fromthe upper and lower ends of the stroke control member 78 are lands 104and 106 respectively, which are positioned on vertically opposite sidesof the circular path 90 and have respective curved surfaces 108 and 110,each of which scalingly engage the outer surface of one or more of thepistons 92 at every position of the pistons 92 along the path 90 anddivide the hollow interior of the stroke control member 78 into leftandrighthand working chambers 112 and 114 (FIG. 2). The intake and exhaustports and 21 of the pump 14 extend vertically through the upper wall ofthe member 78 from the chambers 112 and 114 respectively. The lowerportions of a pair of tubular fittings or conduits 116 and 118 arerespectively sealingly and slidably received in the ports 20 and 21 andthe upper portions of the fittings 116 and 118 are threaded into bores120 and 122 extending vertically through the upper wall of the housing44.

In a manner described below, the strokes or displacements of the pistons92 are adjusted by moving the stroke control member 78 vertically in thehousing 44. For this purpose, the bores 28 and 29 are respectivelylocated in vertical axial alignment in the upper and lower walls of thehousing 44 and the inner ends of the pistons 30 and 32 project into thecavity 46 and are respectively in engagement with flat bearing surfaces128 and 130 on the upper and lower walls of the control member 78. Thus,the pistons 30 and 32 act to shift the member 78 vertically when thebores 28 and 29 are selectively pressurized by operation of the handpump 33 as described above. The control member 78 is guided in itsvertical movement by a pair of spaced, parallel generally rectangularplates 132 and 134 having their outer portions fixed in the walls of thehousing 44 and having inner portions respectively in engagement with theleftand righthand sides and 82 of the control member 78.

In addition to guiding the member 78, the plates 132 and 134 also serveto form portions of working chambers for the pump 16. For this purpose,the plates 132 and 134 respectively have central substantially circularopenings 136 and 138 arranged concentrically to the axis X-X and beingof diameters slightly smaller than those of the plates 62 and 64. Theopenings 136 and 138 are respectively disposed about annular shoulders140 and 142, respectively on the opposed axial ends of the plates 62 and64, the openings 136 and 138 each including a pair of diametricallyopposite, upper and lower lands 144 and 146 forming the only areas ofcontact between the openings and the shoulders. Thus, with reference toFIGS. 3 and 4, it can be seen that the openings 136 and 138 respectivelyof the guide plates 132 and 134 respectively cooperate with theshoulders 140 and 142 of the circular plates 62 and 64 and with the leftand right sides 80 and 82 of the control member 78 to form a left-handpair of substantially fluid-tight annular working chambers 148 and asimilar right-hand pair of annular working chambers 150, the right andleft chambers associated with the plate 62 being in fluid communicationwith the radially outer portions of the slots 100 and the right and leftchambers associated with the plate 64 being in fluid communication withthe radially outer portions of the slots 102. The intakeexhaust port 39of the feedback pump 16 includes a pair of passages 152 (only one shown)extending vertically in the upper portions of the guide plates 132 and134 from the left-hand working chambers 148 to a cross passage 154 inthe upper wall of the housing 44. The passage 42 is also in the upperwall of the housing 44 and interconnects the cross passage 154 with thepiston bore 28. Similarly the intake-exhaust port 40 of the pump 16includes a pair of passages 156 extending vertically in the lowerportions of the plates 132 and 134 from the right-hand working chambersto a cross passage 158 in the lower wall of the housing 44, the passage41 interconnecting the cross passage 158 with the piston bore 29.

The axial forces acting on the circular plates 62 and 64 during pumpingoperation, due to the pressure existing in the left and right workingchambers 112 and 114 of the pump 14, are balanced by the action ofaxially opposed left-hand groups and axially opposed righthand groups ofsmall pistons 160 and 162 respectively located in bores arranged insubstantially semi-circular paths in the axial inner ends of the rightand left sides of each of the bearing end caps 58 and 60, as viewed fromthe left end of shaft 56 in FIG. 3. The left and right groups of pistons160 and 162 respectively have their inner axial ends disposed forrespectively engaging the left and right sides of the outer axial endsof the thrust rings 74 and 76. Thus, the left groups of pistons 160 arein axial alignment with and on the opposite sides of the chamber 112 andthe right groups of pistons 162 are in axial alignment with and on theopposite sides of the chamber 114. A fluid passage 163, shown branchedfrom linev27, interconnects the left working chamber 112 with grooves164 located in the end caps 58 and 60 on opposite sides of the chamber112 and being in fluid communication with the outer, axial endsof thepistons 160. A further fluid passage 165, shown branched from line 26,interconnects the t .5 right working chamber 114 with grooves 166located in the end caps 58 and 60 on opposite sides of the chamber 114and being in fluid communication with the outer axial ends of thepistons 162.

A description of the operation of the power steering system It) followsand it is to be noted that in this description it is assumed that thesystem 10 is completely charged with hydraulic fluid and that right andleft steering is respectively effected by extension and retraction ofthe steering motor 24.

The drive shaft 56 of the steering and feedback fluid control pumps 14and 16, respectively, is driven continuously, in the direction of thearrows (FIGS. 2 and 4), during operation of the vehicle engine 18. Whenno steering is taking place, the hand pump 33 is, of course,

not being operated and fluid is blocked in the lines 37 and 38 and holdsthe stroke control pistons 30 and 32 in central positions wherein thepistons support the stroke con-trol member 78 centrally in the housing44. The pumps ,14 and 16 are then in neutral, zerodisplacementconditions wherein the pumping elements 92 of the pump 14 and thepumping'elements 96 and 98 of the pump 16 trace respective circularpaths about an axis concentric with that of the drive shaft 56. When thepaths are so positioned, there is no relative movement between adjacentpumping elements 92, between the pumping elements 96 and the slots 100,nor

between the pumping elements 98 and the slots 102.

Thus, no fluid is displaced by the pumps 14 and 16.

If it is desired to steer the vehicle to the right, the pump 14 isplaced in stroke in one ofa first set of active conditions wherein fluidis pumped from the line 26 to the line27 to cause extension of thesteering motor 24. This is accomplished by turning the steering wheel 34clockwise to drive the hand pump 33 to displace fluid from the line 38to the line 37 to cause the stroke control pistons 30 and 32 to shiftdownwardly and position the stroke control member 78 below its centralposition a distance determined by the amount of fluid displaced by thepump 33, the member 78 being illustrated in its lowermost position. Thepumping elements 92, 96 and 98 now are positioned totrace-respectivecircular paths which are eccentric toand below the driveshaft 56. As adjacent elements-92 move downwardly in the right chamber114 the spaces therebetween open and as adjacent elements 92 moveupwardly in the left chamber 112, the spaces therebetween close. Thus,fluid is pumped from the right to the left chamber and hence from theline 26 to the line 27 to cause extension of the steering cylinder 24.It is to be noted that the amount of fluid displaced by the pump 14depends upon the eccentricity of the. axis of the path of the elements92, the fluctuations of the spaces between adjacent elements 92increasing with increasing eccentricity.

The feedback fluid control pump 16 is placed into stroke simultaneouslywith the pump 14 since the pumping elements 96 and 98 respectively moveradially outwardly in the slots 100 and 102 concurrently with thedownward movement of the pumping elements 92 and move radially inwardlyin the slots 100 and 102, concurrently with the upward movement of thepumping elements 92 thus causing fluid to be pumped from the workingchambers 148 to the working chambers 150 and hence from the line 37 tothe line 38 to partially nullify the effect of the hand pump 33 on thestroke control pistons 30 and 32. When the right turn is completed, thesteering wheel 34 is again held stationary, thus deactivating the pump33. The pump 16 continues to displace fluid from the line 37 to the line38 until the stroke control pistons 30 and 32 again positionthe strokecontrol member 78 centrally in the housing44 to dispose the paths of thepumping elements 92, 96 and 98 concentrically with the shaft 56 and onceagain establish the neutral, zero-displacement conditions in the pumps14 and 16.

The vehicle may be steered to the left by placing the pump 14 in strokein one of a second set of active conditions wherein fluid is transferredfrom the line 27 to the line 26 to cause retraction of the steeringmotor 24. This is accomplished in much the same manner as the operationfor steering to the right, however, now the steering wheel 34 is turnedcounterclockwise to drive the pump 33 to transfer fluid from the line 37to the line 38 and cause the pistons 30 and 32 to position the strokecontrol member 78 above its control position. The pumping elements 92,96 and 98 again trace paths which are eccentric to the shaft 56,however, now the space between adjacent elements 92 progressively closeand open respectively as the elements 92 move downwardly in the rightchamber 114 and upwardly in the left chamber 112. Thus, fluid is pumpedfrom the line 26'to the line 27 to cause retraction of the steeringmotor 24. The pumping elements 96 and 98 move with the elements 92 andact in the slots 100 and 102 to transfer fluid from the working chamberto the working chamber 148, and hence from the line 38 to the line 37 inopposition to the action of the hand pump 33. Thus, as describedrelative to right-hand steering, the feedback pump 16 acts to restoreitself and the pump 14 to their neutral conditions once the driving ofthe hand pump 33 has stopped.

The axial fluid pressure loads transferred to the bearing assemblies 70and 72, during rightand'left-hand turning operations when the leftandright-hand working chambers 112 and 114 are respectively pressurized,are balanced by fluid pressure delivered from the chamber 112 to thegroups of pistons 162 via the line 163 and the grooves 166 and by fluidpressure delivered from the chamber 114 to the groups ofpistons via theline 165 and the grooves 164.

Thus, it can be seen that since the pumps 14 and 16 are driven at thesame speed and their strokes are adjusted simultaneously by proportionalamounts, the displacement of the smaller pump 16 is always a fixedproportion, less than one, of the larger pump 14 when the pumps are inone or the other of their sets of active conditions. Further, it can beseen that since the amount of fluid displaced by thehand pump 33determines the position of the stroke control member 78, thedisplacements of the pumps 14 and 16 are a direct proportion of theamount of fluid pumped by the hand pump 33. Additionally, it can be seenthat a steering system is provided wherein no valving is used. Thesefeatures are important in that they give stability and responsiveness tothe system 10 as well as give the operator a feel" for theresponsiveness of the system.

1 claim:

1. A pump assembly comprising: a housing; a drive shaft rotatablymounted in said housing; pumping element mounting means includingcentrally-located means defining a cylindrical ring-shaped openingencircling said shaft and a pair of circular end plate means fixed forconcentric rotation with said shaft at spaced locations thereon inclosing relationship to the axially opposite ends of said ring-shapedopening; said centrally-located means being mounted in said housing formovement in a path crosswise to said shaft, the axis of said ring-shapedopening being located such that it lies on the axis of said shaft whenthe ring-shaped opening is at an intermediate position relative to saidshaft; a plurality of equiangularly-spaced, radially extending guideslots arranged in opposed pairs in the opposed surfaces of said circularplates; a plurality of pumping elements having the shapes of cylindricalring segments being located in said ring-shaped opening respectivelybetween each pair of opposed guide slots; a plurality of guide slipperslocated one each in each of said guide slots, the pair of slippers inthe opposed pairs of guide slots being pivotally connected to theopposite sides of the pumping element located therebetween; saidcentrally located means forming a pair of fluid chambers in fluidcommunication with the spaces between said pumping elements, thechambers being separated by diametrically-spaced lands located in linewith said path; first and second intake-exhaust ports being in fluidcommunication respectively with said pair of fluid chambers;displacement control means operatively connected to said centrallylocated means for selectively shifting the latter along said path; saidcircular end plate means each including an annular shoulder at theradially outer ends of said guide slots; a pair of guide plate meansfixed to said housing and respectively having substantially circularopenings disposed concentrically about and spaced from said annularshoulders; said guide plate means embracing said end plate means andsaid centrally located means and cooperating therewith to form a pair offluid chambers in fluid communication with the radially outer ends ofsaid guide slots; and a pair ofland means in each of said circularopenings extending radially inwardly from diametrically oppositelocations in line with said path into sealing engagement with saidshoulder thus dividing said pair of chambers into first and secondhalves and first and second intake-exhaust port means respectively beingin fluid communication with the first and second halves of said pair ofchambers.

2. A pump assemblycomprising: a housing; a drive shaft rotatably mountedin said housing; pumping element-mounting means includingcentrally-located means defining a cylindrical ring-shaped openingencircling said shaft, and a pair of circular end plate means fixed forconcentric rotation with said shaft at spaced locations in closingrelationship to the axially opposite ends of said ring-shaped openings;a plurality of opposed pairs of equiangularly-spaced guide slotsextending radially in the axially opposed surfaces of said plates; aplurality of ring segment-like pumping elements located in saidring-shaped opening, one being between each opposed pair of guide slots;slipper means pivotally connected to each of said pumping elements andbeing slidably received in said guide slots for constraining saidelements for rotation with said shaft and for radial movement in saidslots; said centrallylocated means being mounted in said housing formovement along a straight path to either side of said shaft from anintermediate position wherein said ring-shaped opening is concentricwith said shaft; said centrallylocated means forming a fluid chamber influid communication with the spaces between said pumping elements; saidcentrally-located means including land means at diametrically oppositelocations relative to said ring-shaped opening in line with said pathand cooperating with the radial outer portions of said pumping elementsto divide said fluid chamber into first and second halves; first andsecond intake-exhaust ports being in fluid communication respectivelywith said first and second chamber halves; and displacement controlmeans operatively connected to said centrallylocated means forselectively moving the latter along said path; said shaft beingrotatably mounted in said housing through means of a pair of bearingassemblies respectively received in cylindrical openings in oppositesides of said housing and having opposed, axial inner ends in engagementwith said circular end plate means; a pair of cap members respectivelyreceived in closing relation to said cylindrical housing openings andhaving axial inner ends respectively adjacent the axial outer ends ofsaid bearing assemblies; a plurality of pressure-balancing pistonsshiftably mounted in bores arranged circularly in each of said capmembers in axial alignment with the axial outer ends of said bear ingmembers for movement against said bearing mem' bers for counteractingaxial pressure loads imposed on said bearing members by forces acting onsaid circular end plates; the balancing pistons in each cap member beingdivided into first and second groups respectively in axial alignmentwith said pair of fluid chambers; and fluid passage means respectivelyconnecting said pair of fluid chambers to said first and second groupsof pistons.

3. A pump assembly comprising: a housing; a drive shaft rotatablymounted in said housing; pumping element-mounting means includingcentrally-located means defining a cylindrical ring-shaped openingencircling said shaft and a pair of circular end plate means fixed forconcentric rotation with said shaft at spaced locations thereon inclosing relationship to the axially opposite ends of said ring-shapedopening; said centrally-located means being mounted in said housing formovement in a path cross-wise to said shaft, the axis of saidring-shaped opening being located such that it lies on the axis of saidshaft when the ring-shaped opening is at an intermediate positionrelative to said shaft; a plurality of .equiangularly-spaced, radiallyextending guide slots arranged in opposed pairs in the opposed surfacesof said circular plates; a plurality of pumping elements having theshapes of cylindrical ring segments being located in said ring-shapedopening respectively between each pair of opposed guide slots; aplurality of guide slippers located one each in each of said guideslots, the pair of slippers in the opposed pairs of guide slots beingpivotally connected to the opposite sides of the pumping element locatedtherebetween; said centrally located means forming a pair of fluidchambers in fluid communication with the spaces between said pumpingelements, the chambers being separated by diametrically-shaped landslocated in line with said path; first and second intake-exhaust portsbeing in fluid communication respectively with said pair of fluidchambers; displacement control means operatively connected to saidcentrally located means for selectively shifting the latter along saidpath; said shaft being rotatably mounted in said housing through meansof a pair of bearing assemblies respectively received in cylindricalopenings in opposite sides of said housing and having opposed, axialinner ends in engagement with said circular end plate means; a pair ofcap members respectively received in closing relation to said cylin- 10forces acting on said circular end plates; the balancing pistons in eachcap member being divided into first and second groups respectively inaxial alignment with said pair of fluid chambers; and fluid passagemeans respectively connecting said pair of fluid chambers to said firstand second groups of pistons.

1. A pump assembly comprising: a housing; a drive shaft rotatablymounted in said housing; pumping element mounting means includingcentrally-located means defining a cylindrical ringshaped openingencircling said shaft and a pair of circular end plate means fixed forconcentric rotation with said shaft at spaced locations thereon inclosing relationship to the axially opposite ends of said ring-shapedopening; said centrally-located means being mounted in said housing formovement in a path crosswise to said shaft, the axis of said ring-shapedopening being located such that it lies on the axis of said shaft whenthe ring-shaped opening is at an intermediate position relative to saidshaft; a plurality of equiangularly-spaced, radially extending guideslots arranged in opposed pairs in the opposed surfaces of said circularplates; a plurality of pumping elements having the shapes of cylindricalring segments being located in said ring-shaped opening respectivelybetween each pair of opposed guide slots; a plurality of guide slipperslocated one each in each of said guide slots, the pair of slippers inthe opposed pairs of guide slots being pivotally connected to theopposite sides of the pumping element located therebetween; saidcentrally located means forming a pair of fluid chambers in fluidcommunication with the spaces between said pumping elements, thechambers being separated by diametrically-spaced lands located in linewith said path; first and second intake-exhaust ports being in fluidcommunication respectively with said pair of fluid chambers;displacement control means operatively connected to said centrallylocated means for selectively shifting the latter along said path; saidcircular end plate means each including an annular shoulder at theradially outer ends of said guide slots; a pair of guide plate meansfixed to said housing and respectively having substantially circularopenings disposed concentrically about and spaced from said annularshoulders; said guide plate means embracing said end plate means andsaid centrally located means and cooperating therewith to form a pair offluid chambers in fluid communication with the radially outer ends ofsaid guide slots; and a pair of land means in each of said circularopenings extending radially inwardly from diametrically oppositelocations in line with said path into sealing engagement with saidshoulder thus dividing said pair of chambers into first and secondhalves and first and second intake-exhaust port means respectively beingin fluid communication with the first and second halves of said pair ofchambers.
 2. A pump assembly comprisIng: a housing; a drive shaftrotatably mounted in said housing; pumping element-mounting meansincluding centrally-located means defining a cylindrical ring-shapedopening encircling said shaft, and a pair of circular end plate meansfixed for concentric rotation with said shaft at spaced locations inclosing relationship to the axially opposite ends of said ring-shapedopenings; a plurality of opposed pairs of equiangularly-spaced guideslots extending radially in the axially opposed surfaces of said plates;a plurality of ring segment-like pumping elements located in saidring-shaped opening, one being between each opposed pair of guide slots;slipper means pivotally connected to each of said pumping elements andbeing slidably received in said guide slots for constraining saidelements for rotation with said shaft and for radial movement in saidslots; said centrally-located means being mounted in said housing formovement along a straight path to either side of said shaft from anintermediate position wherein said ring-shaped opening is concentricwith said shaft; said centrally-located means forming a fluid chamber influid communication with the spaces between said pumping elements; saidcentrally-located means including land means at diametrically oppositelocations relative to said ring-shaped opening in line with said pathand cooperating with the radial outer portions of said pumping elementsto divide said fluid chamber into first and second halves; first andsecond intake-exhaust ports being in fluid communication respectivelywith said first and second chamber halves; and displacement controlmeans operatively connected to said centrally-located means forselectively moving the latter along said path; said shaft beingrotatably mounted in said housing through means of a pair of bearingassemblies respectively received in cylindrical openings in oppositesides of said housing and having opposed, axial inner ends in engagementwith said circular end plate means; a pair of cap members respectivelyreceived in closing relation to said cylindrical housing openings andhaving axial inner ends respectively adjacent the axial outer ends ofsaid bearing assemblies; a plurality of pressure-balancing pistonsshiftably mounted in bores arranged circularly in each of said capmembers in axial alignment with the axial outer ends of said bearingmembers for movement against said bearing members for counteractingaxial pressure loads imposed on said bearing members by forces acting onsaid circular end plates; the balancing pistons in each cap member beingdivided into first and second groups respectively in axial alignmentwith said pair of fluid chambers; and fluid passage means respectivelyconnecting said pair of fluid chambers to said first and second groupsof pistons.
 3. A pump assembly comprising: a housing; a drive shaftrotatably mounted in said housing; pumping element-mounting meansincluding centrally-located means defining a cylindrical ring-shapedopening encircling said shaft and a pair of circular end plate meansfixed for concentric rotation with said shaft at spaced locationsthereon in closing relationship to the axially opposite ends of saidring-shaped opening; said centrally-located means being mounted in saidhousing for movement in a path cross-wise to said shaft, the axis ofsaid ring-shaped opening being located such that it lies on the axis ofsaid shaft when the ring-shaped opening is at an intermediate positionrelative to said shaft; a plurality of equiangularly-spaced, radiallyextending guide slots arranged in opposed pairs in the opposed surfacesof said circular plates; a plurality of pumping elements having theshapes of cylindrical ring segments being located in said ring-shapedopening respectively between each pair of opposed guide slots; aplurality of guide slippers located one each in each of said guideslots, the pair of slippers in the opposed pairs of guide slots beingpivotally connected to the opposite sides of the pumping element locAtedtherebetween; said centrally located means forming a pair of fluidchambers in fluid communication with the spaces between said pumpingelements, the chambers being separated by diametrically-shaped landslocated in line with said path; first and second intake-exhaust portsbeing in fluid communication respectively with said pair of fluidchambers; displacement control means operatively connected to saidcentrally located means for selectively shifting the latter along saidpath; said shaft being rotatably mounted in said housing through meansof a pair of bearing assemblies respectively received in cylindricalopenings in opposite sides of said housing and having opposed, axialinner ends in engagement with said circular end plate means; a pair ofcap members respectively received in closing relation to saidcylindrical housing openings and having axial inner ends respectivelyadjacent the axial outer ends of said bearing assemblies; a plurality ofpressure-balancing pistons shiftably mounted in bores arrangedcircularly in each of said cap members in axial alignment with the axialouter ends of said bearing members for movement against said bearingmembers for counteracting axial pressure loads imposed on said bearingmembers by forces acting on said circular end plates; the balancingpistons in each cap member being divided into first and second groupsrespectively in axial alignment with said pair of fluid chambers; andfluid passage means respectively connecting said pair of fluid chambersto said first and second groups of pistons.